The sialic acids are the outermost sugars on cell surface and secretory glyco-proteins and glycolipids of mammalian cells. It is now known that these molecules can be substituted with O-acetyl or O-lactyl groups at the 4, 7, 8, and 9 positions and that such modifications vary widely in an organ and species-specific fashion. While many biological roles have been attributed to the sialic acids, the significance of such substitutions is not clear in most situations. I plan to study the biosynthesis and regulation of O-substituted sialic acids as a first step towards exploring their biological significance. Methods are proposed for the idenfification of continuous tissue culture cell lines that synthesize different types of O-substituted sialic acids. Such cell lines will then be studied by metabolic labeling with radioactive precursors, to understand the mechanisms, subcellular sites and kinetics of O-acetylation. In addition, the O-acetyltransferase enzymes from specific tissue sources will be identified, purified and characterized. Factors modifying the enzymatic reaction in vitro, as well as those modifying O-acetylation in the intact cell will be identified, in order to understand the mechanisms of regulation of O-acetylation. A search will also be made for specific de-O-acetylases. With the information and materials obtained from these studies, it will be possible to explore the biological roles of O-acetylated sialic acids, and to elucidate pathological processes in which they may be involved. Preliminary evidence suggests these substitutions have effects on such diverse processes as alternate pathway complement activiation, viral neuraminidase action, the virulence of E. Coli in neonatal meningitis and the difference between normal and malignant colinic tissues.